Spectator stands or terrace installations with seats, the so-called gradin system, are generally permanent. They are usually made of wood, steel or concrete construction. In certain cases they are made of prefabricated elements, which are assembled on the site. Examples of such permanent tier systems are described in Swedish patent specifications 73 676 and 206 022 and also in U.S. Pat. No. 2,524,769.
However, installations are often needed which can be quickly set up and dismantled and which consist of portable parts. For example in circuses such systems have been used for a long time. The constituent elements, are often very heavy. Its is desirable that no element should weigh more than 20 to 25 kg. The greatest weakness is, however, that the joints are not reliable. In general they use connecting elements of the types with hooks and eyes, pins, ball and socket joints, screws and nuts and the like. A disadvantage of such connecting elements is that they can slacken (there is no screw connection that is completely safe) and come apart during handling. In addition the usual systems are more or less unstable. Furthermore they are often troublesome to erect. An example of a portable stand or gradin system intended to be assembled with the aid of pins is the construction which is described in Swedish patent specification No. 424 210.